1. Technical Field
The present invention relates to piezoelectrics, and piezoelectric elements and piezoelectric actuators using the piezoelectrics.
2. Related Art
Piezoelectric elements are used in a variety of applications, such as, piezoelectric actuators, pressure sensors, ultrasonic devices, liquid ejection heads, ink jet printers and the like. Piezoelectric elements generally have a structure in which a piezoelectric layer formed from piezoelectrics is sandwiched between electrodes. As the piezoelectrics to be used, Pb(Zr, Ti)O3 (lead zirconate titanate: PZT) may be typically used.
In an attempt to improve various piezoelectric characteristics of such piezoelectrics, for example, piezoelectrics in which at least a part of Zr and Ti at the B site (to be described below) of PZT is replaced with Nb have been proposed, as described in, for example, Japanese Laid-open Patent Application JP-A-2005-100660. Such piezoelectrics exhibits higher reliability when formed into piezoelectric elements, compared to PZT.
Perovskite type compounds are compounds whose compositional formula is generally expressed by ABX3. Perovskite type compounds, when expressed by such formula, have a structure in which, when crystallized, a positive ion at the position of A element (hereafter referred to as the A site) bonds with 12 negative ions X as ligands, and a positive ion at the position of B element (hereafter referred to as the B site) bonds with 6 negative ions X as ligands. Such crystal structure is called a perovskite structure. Perovskite type compounds can assume a perovskite structure when crystallized, such that a variety of characteristics, such as, for example, ferroelectricity, pyroelectricity and piezoelectricity can be exhibited.
If perovskite type compounds are formed merely to have a perovskite structure to exhibit only piezoelectricity, the elements may be selected in a relatively wide range of composition ratios. For example, PZT can have a perovskite structure at an arbitrary ratio of Zr and Ti. However, the composition ratio of perovskite type compounds is limited to a very narrow range when they are to be used as piezoelectric elements. PZT is known to have the morphotropic phase boundary (MPB) between rhombohedra and tetragonal phase crystals having perovskite structure near a ratio of Zr/Ti=53:47 in mole ratio. It is known that the piezoelectric constant and the electromechanical coupling coefficient are generally higher in the vicinity of the MPB compositions in perovskite type compounds than in other composition areas, and PZT with compositions in the vicinity of the MPB are mainly used in piezoelectric elements formed from PZT. It is therefore important to control sufficiently the range of compositions of piezoelectric materials when they are to be used for piezoelectric elements.
Due to the reasons described above, in piezoelectrics in which Nb or the like is introduced in the B site to improve the reliability, the compositions of the MPB have not been necessarily clearly understood, and the piezoelectric characteristics would occasionally change when the compositions are changed. In piezoelectrics in which at least a portion of Zr and Ti at the B site in PZT is replaced with Nb, such composition-dependence of the piezoelectric characteristic is not sufficiently understood, and in particular, morphotropic phase boundary (MPB) compositions that are assumed to provide the most excellent piezoelectric characteristics have not been clearly found.